Accurate and precise monitoring of current flow allows the operational state of a circuit to be assessed and the presence of malfunctions to be detected. This is especially useful for circuits that may be powering/controlling more than one device or load. Current will continue to flow through the circuit upon one of the loads malfunctioning. However, the amount of current flowing through the circuit may fluctuate or change due to the malfunctioning load. As such, accurate monitoring of the current flowing through the circuit can reveal the presence of a malfunction. For illustrative purposes, consider the following example, where the function of an automotive circuit is to power all the exterior lights located at the rear of the automobile, such as the left, right and center brake light, along with the turn signal lights. If the left brake light were to malfunction and subsequently act as an open or shorter load, the amount of current flowing through the automotive circuit will fluctuate or change. Through the use of an accurate current monitor, this current change can be detected, alerting the system to the malfunctioning left brake light.
A known current monitoring circuit 5 is depicted in FIG. 1. According to this typical current monitor, a sense resistance Rsense is placed in series with and between the power supply Vbat and load 10. As such, the current going to the load (“load current”) passes through resistance Rsense, which in turn generates a voltage drop across Rsense. By then measuring the voltage drop across the resistance Rsense, the load current can be determined.
As the current monitor 5 is directly connected to the application being monitored, the operational amplifiers OP1 and OP2 that comprise current monitor 5 must be able to accept input voltages up to the magnitude of power supply Vbat of the application. Therefore, as the voltage level of power supply Vbat increases, so must the maximum input voltage of the operational amplifiers OP1 and OP2. Traditional operational amplifiers are only rated to handle a maximum input voltage of 20-30 volts. Consequently, complications arise when typical current monitors, such as that depicted in FIG. 1, are utilized in applications that employ higher voltage power supplies, such as, for example, a 42 V power supply, as common operational amplifiers are not rated to handle these higher voltage levels.
In order to protect the current monitor circuit from excessive voltages, and more specifically, the operational amplifier within the circuit that initially accepts the input signal, it has been necessary to attenuate the input signal to current monitor 5. Attenuation of the input signal is accomplished by an attenuation stage 20, which, as illustrated in FIG. 1, consists of operational amplifiers OP1 and resistors R1-R4 arranged into a differential amplifier configuration. In typical differential amplifiers, the value of resistor R1 equals R3 and the value of resistor R2 equals R4. The differential “amplifier” can be configured to attenuate a signal (as opposed to amplifying it) by choosing values for the resistors such that R1 is greater than R2. In this configuration, the differential amplifier attenuates the input voltages to operable levels before they are applied to the first operational amplifier OP1 within the current monitoring circuit. This technique allows traditional current monitors to be used to monitor circuits that operate at higher voltage levels than those rated for typical operational amplifiers.
After attenuating the signal in order to protect the first operational amplifier within the circuit, current monitor 5 amplifies the signal by means of amplification stage 30. This is necessary as resistance Rsense is designed to be relatively small in value in order to minimize the amount of power lost or dissipated by the current monitor 5. Thus, while voltage V2 and voltage V1 may be quite high in value, the actual voltage drop across resistance Rsense may be quite small. Consequently, the differential voltage applied to operational amplifier OP1, which is equivalent to V2−V1, can be difficult to accurately measure and translate into a current value. To compensate for this small voltage differential, the output voltage VoutA of attenuation stage 20 is fed into amplification stage 30, thereby allowing for an accurate translation of the voltage value into a current value.
The problem that arises with traditional current monitors such as the one described above and depicted in FIG. 1 is that corresponding resistors within the circuit, i.e. R1 and R3, must precisely match in value. If these resistor values are not precisely matched, the voltage levels applied to operational amplifier OP1 will not accurately represent the voltage drop across resistance Rsense. This leads to the common mode gain of the differential amplifier becoming significant in value and overwhelming the sought after voltage difference measured across resistance Rsense. As a result, accurate current detection becomes impossible. Thus, it is required that corresponding resistors, such as R1 and R3, not vary from one another by even 0.1% of their rated value, otherwise a useable voltage signal indicative of the current flow through Rsense is often unobtainable.
Accordingly, for the above-described current monitoring circuit to work properly with a high voltage application, resistors within the attenuation stage must be precisely matched in value. As a result, significant time and expense is required to obtain resistor pairs that match to the degree of precision required by the attenuation stage of the circuit.
Therefore, the inventors hereof have recognized the need for a new circuit and method for monitoring an electrical current within an application.